In acoustic welding, such as ultrasonic welding, two parts to be joined (typically thermoplastic parts) are placed directly below an ultrasonic horn. In plunge welding, the horn plunges (travels toward the parts) and transmits ultrasonic vibrations into the top part. The vibrations travel through the top part to the interface of the two parts. Here, the vibrational energy is converted to heat due to intermolecular friction that melts and fuses the two parts. When the vibrations stop, the two parts solidify under force, producing a weld at the joining surface.
Continuous ultrasonic welding is typically used for sealing fabrics, films, and other parts. In the continuous mode, typically the ultrasonic horn is stationary and the part is moved beneath it. Scan welding is a type of continuous welding in which the plastic part is scanned beneath one or more stationary horns. In transverse welding, both the table over which the parts pass and the part being welded remain stationary with respect to each other while moving underneath the horn or while the horn moves over them.
Many uses of ultrasonic energy for bonding and cutting thermoplastic materials involve ultrasonic horns or tools. A horn is an acoustical tool usually having a length of one-half of the horn material wavelength and made of, for example, aluminum, titanium, or sintered steel that transfers the mechanical vibratory energy to the part. (Typically, these materials have wavelengths of approximately 25 cm (10 in).) Horn displacement or amplitude is the peak-to-peak movement of the horn face. The ratio of horn output amplitude to the horn input amplitude is termed gain. Gain is a function of the ratio of the mass of the horn at the vibration input and output sections. Generally, in horns, the direction of amplitude at the face of the horn is coincident with the direction of the applied mechanical vibrations.
Traditionally, ultrasonic cutting and welding use horns which vibrate axially against a rigid anvil, with the material to be welded or cut being placed between the horn and anvil. Alternatively, in continuous high speed welding or cutting, the horn is stationary while the anvil is rotated, and the part passes between the horn and the anvil. In these cases, the linear velocity of the part is matched with the tangential velocity of the working surface of the rotating anvil.
There are, however, some limitations to this system. Because the part to be welded is continuously passed between the narrow gap formed by the anvil and the horn, compression variations are created due to part thickness nonuniformities. Drag exists between the part and the horn and may cause residual stresses in the welded region. These factors affect the weld quality and strength which, in turn, limit the line speeds. Also, the gap between the rotating anvil and the horn limits the compressible bulk or thickness of the parts to be bonded.
One way to minimize these limitations is to shape the working surface of the horn to attain a progressive convergent or divergent gap depending upon the part. This does not completely solve the problem of moving the material to be bonded past a stationary horn, as an intimate contact is needed for efficient acoustic energy transfer.
The best way to attain high quality and high speed ultrasonic welds is to use a rotary horn with a rotating anvil. Typically, a rotary horn is cylindrical and rotates around an axis. The input vibration is in the axial direction and the output vibration is in the radial direction. The horn and anvil are two cylinders close to each other, rotating in opposite directions with equal tangential velocities. The part to be bonded passes between these cylindrical surfaces at a linear velocity which equals the tangential velocity of these cylindrical surfaces. Matching the tangential velocities of the horn and the anvil with the linear velocity of the material is intended to minimize the drag between the horn and the material. The excitation in the axial direction is similar to that in conventional plunge welding.
U.S. Pat. No. 5,096,532 describes two classes of rotary horn. The patent compares a commercially available rotary horn, manufactured by Mecasonic-KLN, Inc. of Fullerton, Calif. (Mecasonic horn) and a rotary horn described in the '532 patent. FIG. 1 shows a Mecasonic rotary horn and FIG. 2 shows one configuration of the '532 rotary horn. One significant difference between these two types of horns is the width of the radial weld face and the uniformity of amplitude across the radial face.
The Mecasonic horn is a full wavelength horn, having a total length of about 25 cm (10 in) for aluminum and titanium horns. The axial vibration excites the cylindrical bending mode to provide the radial motion, and the mode of vibration depends on Poisson's ratio. (If the Poisson's ratio of the horn material is zero, the radial modes of vibration are not excited.) The radial motion of the weld face is in phase with the excitation, and there are two nodes (where the amplitude of vibration is zero) for the axial motion, and two nodes for radial motion. However, the amplitude of vibration is the highest at the center of the radial weld face and diminishes toward the end, resulting in uneven weld strength. The Mecasonic horn is a partially hollowed cylinder.
The '532 horn is a half wavelength horn, having a total length of about 12.7 cm (5 in) for aluminum and titanium horns. Due to the shape of the horn, the axial vibration provides the radial motion. In this horn, the mode of vibration is independent of Poisson's ratio. The radial motion of the weld face is out of phase with the excitation, and there is only one node, at the geometric center of the weld face. The amplitude of vibration is relatively uniform across the weld face. The shape of the '532 horn differs from that of the Mecasonic horn; the '532 horn is solid, and the Mecasonic horn is a partially hollowed cylinder.
There is a need for an acoustic welding configuration which can weld parts over wide width (such as greater than 12.7 cm).